Virtually all crude oil deposits located in formations below ground lay above a layer of concentrated salt water, or brine, which is thought to be the remnants of ancient lakes and oceans which have partially evaporated with consequent concentration of the dissolved minerals. When a well is drilled into a formation to extract the crude oil, a portion of the brine is also carried along in the production stream. For newly drilled wells, the amount of brine carried along in the production stream may be as little as 5% percent of the total weight of liquid. After many years of operation, however, a well may produce as much as 90% brine and only 10% crude oil. As used herein, crude oil includes all hydrocarbon fractions extracted from underground formations, as understood by those skilled in the art.
The average depth of a well is between approximately 1,500 to 3,000 meters. The well consists of an outer casing surrounding production tubing, at the bottom of which is a cylindrical pump unit. Oil enters the casing at the bottom thereof through perforations in the casing wall. Between the outer casing and production tubing is a concentric volume known as the annulus. In certain instances, the pressure in the formation is sufficient to propel the brine and crude oil mixture through the production tubing to the surface without any separate pumping effort. As the well ages, however, pumping becomes necessary to bring the mixture to the surface. One common device used for pumping the brine and crude oil mixture to the surface is a walking beam pump. This pump is comprised of a beam supported by a center pivot, with a counterweight on one end of the beam and a rod extending to the bottom of the well at the other. Rotary motion applied to a cam connected to the counterweight end of the beam results in a repetitive vertical motion at the rod end of the beam. The rod, called a sucker rod, attaches to the cylindrical pump at the bottom of the well. This rod is made of a solid steel alloy, typically about 1.6 cm in diameter and is constructed by joining together individual rod lengths of about 9.1 meters each using screw connections. Every downstroke of the beam fills the pump, and every upstroke pulls the pump contents up the production tubing. Once the brine and crude oil mixture reaches the surface, it is transported through various tubing members to equipment which is used to separate the brine from the crude oil. This equipment includes free water knock-out tanks, separators and holding tanks, as well as heater treaters which assist in breaking any emulsions of brine and crude oil through either heat or demulsifying chemicals after the mixture is brought to the surface. When the well is extracting a substantial natural gas portion, the material passing up the production tubing is first conducted to a separator and then to a dehydrator unit. The dried natural gas thereafter is conducted to a pipeline for transport and sale.
The metal components comprising the extraction and separation equipment are made from carbon steel, which is susceptible to corrosion, particularly in the presence of brine. The salt concentration of the brine can be as low as about 0.5% by weight. In the West Texas oil fields, the concentration of salt in the brine can range from about 1% to about 12% by weight, though it may reach as high as 18 to 20%. In contrast, the salt content of brine in California oil fields can be as low as about 1.5% by weight, but is more typically in the 6% range.
Corrosion of the extraction and separation equipment is most evident on those components which are subjected to high liquid flow velocities, such as the cylindrical pump at the bottom of the well. The potential economic damage to the oil field well due to corrosion is evident from observation of the sucker rod. Contact with the brine and crude oil mixture, which further may contain dissolved acidic gases such as hydrogen sulfide and carbon dioxide, causes pitting type corrosion along the length of the sucker rod. If the corrosion sufficiently weakens the sucker rod to cause breakage, the well must be shut down and the downhole components completely disassembled. The breakdown and repair time may result in two days of downtime at that particular pump. Since in normal operation with existing corrosion inhibitor additives, the sucker rod may break as often as every two to six months of operation, it is a primary goal to extend the working life of this component.
In previous efforts to inhibit corrosion in the equipment used for extracting the brine and crude oil mixture from underground formations, and in subsequently removing the brine from the crude oil, it was known to use various amine compounds in the equipment. More specifically, certain amine compounds generally known as filming amines have been utilized in this application to inhibit corrosion. Certain filming amine compounds known to have utility in this application are derivatives of imidazoline. The filming amine compounds, which are alkaline, are reactive with acidic species. In the underground formations, gases such as hydrogen sulfide and carbon dioxide, among others, are found which solubilize in the water component of the brine to form acidic species. These acids will react with the amines and thereby adversely affect their activity as a corrosion inhibitor. Further, the active amines as well as the amine salts formed from the reaction with the acidic species tend to facilitate formation of an emulsion of the crude oil and brine. The emulsions contain valuable crude oil which must be further treated using demulsifying chemicals or other separation means, such as heat, to break the emulsion and recover the crude oil.